1. Field of the Invention
The present invention relates to a reflective type liquid crystal display device and method of making the same, and more particularly to a reflective type liquid crystal display device possessing MIM (metal-insulator-metal) element and method of making the same.
2. Description of the Related Art
Recently, the application of liquid crystal display device into the word processor, lap top personal computer, portable television and the like is rapidly increasing. In particular, among the liquid crystal display devices, the reflective type liquid crystal display device for displaying by reflecting the light entering from outside is highly noticed because the back light is not needed, the power consumption is low, and the structure can be made thin and lightweight.
Hitherto, in the reflective type liquid crystal display device, the TN (twisted nematic) type and STN (super twisted nematic) type have been widely employed, but in these types, since 3/4 of the light intensity of the incident light is cut off by the polarizer, the display is dark.
To solve such problem, a display mode is proposed for effectively utilizing all rays of incident light without using polarizer. An example of such display mode is the phase transition type guest-host mode. A display device of such phase transition type guest-host mode is disclosed, for example, by D. L. White and G. N. Taylor: J. Appl. Phys. 45, 4718, 1974. This mode makes use of the cholesteric-nematic phase transition phenomenon by electric field. A microcolor display applying this mode has been proposed by Tohru Koizumi and Tatsuo Uchida in Proceedings of the SID, VOL. 29/2, 157, 1988.
In the TN type, when multiplex drive is effected by voltage equalizing method desired from the standpoints of larger size, higher definition, and lower production cost of the device, sufficient contrast and intermediate tone display may not be obtained. That is, as the display becomes larger in size and higher in definition, the number of pixels increases, and in order to obtain a sufficient contrast in such liquid crystal display device, a steeper property is required as the optical characteristic change of the liquid crystal to voltage changes. However, when the optical characteristic change of the liquid crystal becomes steep, it is hard to obtain the intermediate tone display. Accordingly, in order to obtain a high contrast while keeping moderate optical characteristic changes relative to voltage changes, active drive employing nonlinear elements, that is, three-terminal elements such as TFT (thin-film-transistor) elements, and two-terminal elements such as diode elements and MIM (metal insulator metal) elements is executed. In particular, in the reflective type liquid crystal display device, the two-terminal elements having a greater aperture rate as compared with the three-terminal elements, that is, a greater rate of the active moving picture area to the display picture area is preferably used.
FIG. 16 is a plan view showing a region 7 of an insulating substrate 1 corresponding to one pixel in a conventional reflective type liquid crystal display device. Incidentally, in FIG. 16 which is a plan view, an MIM element 4 is indicated by hatching. The region 7 corresponding to one pixel is constituted by comprising a pixel electrode 2 responsible for display being formed on the insulating substrate 1, and a signal wiring 3 and MIM element 4 not responsible for display. The MIM element 4 shown in hatched area is a nonlinear switching element of two-terminal element for connecting the pixel electrode 2 and signal wiring 3.
FIG. 17 is a sectional view of cutting off the MIM element 4 shown in FIG. 16 on a cut-off line A--A. The MIM element 4 is composed of a lower electrode 3a which is a part of the signal wiring 3 formed on the insulating substrate 1, an insulator 5, and an upper electrode 6. On the lower electrode 3a, the insulator 5 is formed, and the upper electrode 6 is formed on the insulator 5. On the insulating substrate 1 including a part of the upper electrode 6, the pixel electrode 2 is formed. The insulating substrate 1 having such MIM elements 4 disposed corresponding to plural pixels is glued with a counter substrate forming a counter electrode through a liquid crystal layer.
Generally is known the MIM element using tantalum (Ta) as the lower electrode 3a, using tantalum oxides (TaO.sub.x) as the insulator 5, and using chromium (Cr), titanium (Ti) or aluminum (A1) as the upper electrode 6. The display device using such MIM element is disclosed, for example, in the Japanese Unexamined Patent Publication (KOKAI) No. JP-A 3-35223, 3-41420, 2-308138, 2-304534, 3-149526, 2-83538, 3-296024, and 4-114132.
FIG. 18 is an equivalent circuit diagram of a reflective type liquid crystal display device using the MIM elements 4. The signal electrode wirings (data wirings) are indicated as D1, D2, D3, --(or D collectively), and scanning electrode wirings (address wirings) as S1, S2, --(or S collectively), and the individual wirings are arranged so as to intersect alternately by a plurality each. At the intersections of these wirings, the liquid crystal cell LC and MIM element 4 are connected in series. The current flowing in the MIM element 4 depends on the interface characteristic of the lower and upper electrodes 3a, 6 and the insulator 5, and approximately, I=aV.sup.n is given. Therefore, the nonlinearity of the MIM element 4 can be evaluated by the value of n. Herein, I denotes the current, v is the voltage, and a and n are constants.
FIG. 19 is an equivalent circuit diagram of one pixel of the reflective type liquid crystal display device using the MIM elements 4. The resistance of the MIM element 4 is expressed as R.sub.NL, and the capacity is C.sub.NL. The resistance of the liquid crystal cell LC is expressed as R.sub.LC, and the capacity is C.sub.LC. The MIM element 4 is expressed by a parallel circuit of resistance R.sub.NL and capacity C.sub.NL, and the liquid crystal cell LC is expressed by a parallel circuit of resistance R.sub.LC and capacity C.sub.LC. The MIM element 4 comprises a nonlinear voltage-current characteristic, and the resistance R.sub.NL changes suddenly by the voltage at both ends.
A selection pulse of amplitude V1 is applied in every period T to the scanning electrode wiring S, and data signals for determining the orientation state of liquid crystal and determining the display state are applied to the signal electrode wiring D in an amplitude .+-.V2. When the selection pulse is applied to the scanning electrode wiring S, the voltage (V1.+-.V2) applied to the selected pixel is divided in capacity, and the voltage applied to the MIM element 4 is expressed as V.sub.NL =C.sub.LC /(C.sub.LC +C.sub.NL).multidot.(V1.+-.V2). Herein, when the capacity C.sub.NL of the MIM element 4 is set sufficiently small as compared with the capacity C.sub.LC of the liquid crystal cell LC, almost all voltage is applied to the MIM element 4, and the MIM element 4 is set in ON state having a low resistance, and the electric charge corresponding to the display data is written into the capacity C.sub.LC of the liquid crystal cell LC. The written data is held by the capacity C.sub.LC of the liquid crystal cell LC for the duration until selected again and new data is written.
In the region 7 of the insulating substrate 1 corresponding to one pixel of a reflective type liquid crystal display device using the MIM elements 4, the active region functioning for screen display is a forming region of the pixel electrode 2 except for the overlapping portion with the upper electrode 6. The nonactive region not functioning for screen display is a region other than the active region, and includes the signal wiring 3, gap of pixel electrode 2 and signal wiring 3, and gap of adjacent pixel electrodes 2. In this case, the aperture rate is 70 to 50%.
Recently, along with the trend of higher definition of the liquid crystal display device, a higher density is required for the number of pixels. As a result, the aperture rate is lowered, the display becomes darker, and the display quality is lowered, so that various problems may be caused. This is because the rate of the active region is lowered as the minimum required area as the MIM element 4 and signal wiring 3 is limited if the number of pixels is heightened in density.